The present invention relates to a passive dosimeter for the detection of radon and/or thoron and their daughters, the dosimeter being of the type composed of a diffusion chamber having two end faces, one of which is sealed by a nuclear track detector foil or an integrating .alpha.-particle detector, and the other of which is sealed by a filter transmissive for radon and/or thoron, and by means of which the radon and thoron decay and the resultant daughters are determined.
The effect of radon/thoron and their daughters contributes substantially to natural radiation exposure. These .alpha.-radionuclides are present in the free atmosphere and enter the lungs after an .alpha.-decay as free ions and/or attached to aerosols. In correspondence with the radium and thorium content in the soil and in building materials, the average inhalation dose of the population due to radon and daughters amounts to 775 mrem/year, based on the entire lung. In closed dwellings, but above all in mines, 10 to 100 times this lung exposure can occur under certain circumstances.
When measuring radon and daughters, one is confronted with the task of determining the radiation burden of, above all, the daughters, since the noble gas radon .sup.222 Rn remains in the lung only for a short period of time, whereas the shortlived daughters bound to aerosols are left in the bronchial cavity.
Tables 1 and 2 below set forth the decay series for radon and thoron.
TABLE 1 ______________________________________ E.sub..alpha. Name Isotope Half-Life Value Type of Radiation Mev ______________________________________ Radon 222.sub.Rn 3.823 d .alpha. 5.49 RaA 218.sub.Po 3.05 min .alpha. 6.00 RaB 214.sub.Pb 26.8 min .beta.,.gamma. -- RaC 214.sub.Bi 19.7 min .beta.,.gamma. -- RaC 214.sub.Po 164 .mu.s .alpha. 7.69 RaD 210.sub.Pb 21 y .beta.,.gamma. -- ______________________________________
TABLE 2 ______________________________________ E Name Isotope Half-Life Value Mev ______________________________________ Thoron 220.sub.Rn 55.65 s 6.29 Th A 216.sub.Po 0.15 s 6.78 Th B 212.sub.Pb 10.6 h -- Th C' 212.sub.Po 0.3 .mu.s 8.78 Th C" 208.sub.Tl 3.1 min -- ______________________________________
Measurement of radon/thoron and daughters is effected according to the present state of the art primarily with battery-operated instruments which take in air via a filter and measure the daughters separated on the filter. In this connection, three methods are utilized essentially, wherein a specific amount of air is pumped through a filter, as described in IAEA Safety Series No. 43, "Manual on Radiological Safety in Uranium and Thorium Mines and Mills":